1. Field of the Invention
The invention relates to a method of plating on a glass substrate and a method of manufacturing a magnetic recording medium using the method of plating. The method of plating is useful in particular for the formation of an electroless plating film on a glass substrate for a magnetic recording medium installed in a hard disk drive.
2. Description of the Related Art
In magnetic recording media (hard disks: HDs) installed in hard disk drives (HDDs) used in external storage devices of computers, a glass substrate exhibiting superior flatness and strength than an aluminum alloy substrate has become more popular to meet demands for large storage capacity suited to installation in AV appliances, and for higher recording density on decreased diameter HDs.
An aluminum alloy substrate normally has a NiP plating film formed on it by means of an electroless plating method. For a glass substrate, on the other hand, forming a plating film on it with satisfactory adhesiveness by means of the electroless plating method, has been technologically difficult.
A technique may be possible, in which an adhesion layer is formed on a glass substrate by sputtering. According to this technique, the adhesive layer is composed of a layer containing titanium or chromium that exhibits relatively good adhesion with glass in the metallic substances. On the adhesion layer, a desired film is laminated. The technique, however, requires repetition of processing for each of multiple laminations, and therefore finds difficulty in cost reduction as compared with a plating method, which allows batch processing. Thus, the technique is not suited to form a thick film. In particular, a perpendicular magnetic recording medium, which is a subject of active development, needs a relatively thick layer, a soft magnetic underlayer having a thickness of 0.3 to 3.0 μm. Such a thick layer can only be obtained at high cost when deposition is performed by a sputtering method.
A method to form a plating film on a surface of a glass substrate by means of an electroless plating method has been proposed in Japanese Unexamined Patent Application Publication No. 2000-163743, in which an adhesion layer is formed of a silane coupling agent and an electroless plating film is formed on the adhesion layer.
In this method, the ethoxyl group or methoxyl group in the silane coupling agent becomes a silanol group when the silane coupling agent dissolves in water, and the silanol group bonds through the hydrogen bond to the hydroxyl group or silanol group generated by activation of the surface of the glass substrate. After a dehydration treatment is performed, adhesion through a firm chemical bond is accomplished. Consequently, a plating film with good adhesiveness is yet obtained on a smooth substrate surface without utilizing an anchoring effect by surface coarsening as in a sensitization-activation method.
Japanese Unexamined Patent Application Publication No. H8-39728 discloses a method in which a silane coupling agent solution is applied to a glass epoxy substrate for a printed wiring board and dried. The substrate then is plated with a metal in the presence of a metallizing catalyst.
In the above-described method using the adhesion layer of silane coupling agent, it is important that the state of combination between the hydroxyl groups on the glass substrate and the silanol groups of the silane coupling agent is a chemical bond. The existence of a portion not chemically bound, but in a simple adhesion of a hydrogen bond, has been found to cause poor adhesion, blisters, and peeling of the plated film. The state of combination can be made into chemical bonds in some extent by an activation treatment on the surface of the glass substrate, but it is difficult to entirely establish chemical bonding.